The goal of the proposed research is to further the understanding of retrovirus structure and mode of assembly. Primarily avian viruses will be investigated, but in some instances comparative studies with mammalian viruses will also be done. The major thrust of the work will be directed toward three areas, in all of which experiments already are in progress and in which advances have been made recently in this laboratory. The first is the interaction of viral structural proteins, as well as other proteins, with the lipid bilayer, as probed by the lipid-protein cross-linking technique that we have developed. The second area is proteolytic cleavage of viral proteins. The mechanism and specificity of cleavage by the viral protease p15 will be analyzed in detail, and the role of cellular proteases in cleavage of viral proteins will be assessed. The third and related area of investigation comprises the several cases where avian retrovirus assembly does not take place or is aberrant. Unraveling the blocks to proper assembly will contribute to understanding of its mechanism.